Another concern of mine is how much fresh air does a boat have naturally flowing through it for the occupants.
We always have at least one hatch cracked open no matter how cold it is when we're sleeping. And never a heating device of any kind.
One other piece of advice as far as air for sleeping goes, "NEVER LET A POWER BOATER BACK HIS GIN MILL INTO YOUR TRANSOM" if you should find yourself on a long dock.
Three of us in our boat didn't pay too much attention to the gin mill sitting transom to transom behind us that evening and almost didn't wake up the next morning. We tied up long before this guy arrived. Seems the guy operating this behemoth behind us decised to run his engines for a good while early the next morning and filled our cabin with CO as our companionway was open. I woke up the next morning ready to puke my guts out and started yelling at everyone in our cabin to get up. The #$%$^&^%& was just leaving in a cloud of smoke and didn't know and couldn't care less if we were dead or alive. If I had been able to jump aboard his boat before he left, I would have cheerfully split his head open with a winch handle and worried about the consequences later.
Under no circumstances would I ever let one of these morons pull a transom to transom stunt with my boat again. You're looking at a possible death sentence if he decides to fire up his boat before you are awake in the morning, even if he tells you he's not leaving early. And trust me .............. couldn't care less.
Reason #99 why I hate power boaters.